Motion: Part 1
July 1, 2021

Things move relatively. You may be sitting down motionless with respect to Earth’s surface, while also orbiting at 30,000 m/s with respect to the Sun. Relativity is about two kinds of motion.

Intrinsic motion is an object's motion with respect to space itself. It is impossible to truly observe intrinsic motion due to the observer having motion of its own. Apparent motion is the observed motion with respect to another entity. The delineation of these two motions is essential.

Although it would seem intrinsic motion is the motion that physical equations would pertain to, General Relativity has shown that this is not the case. Take for example, Newton’s First Law:

Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.

Translation - "An object at rest will remain at rest unless acted upon by an external and unbalanced force. An object in motion will remain in motion unless acted upon by an external and unbalanced force"

Gravity Is Not Intrinsic

Newton's First Law basically says the forces accelerate/decelerate motion, but he did not say what kind of motion. Today, a force is defined as something that can change an object's motion with respect to space, changing intrinsic motion. So it may come as a surprise that gravity is not a force by this definition. In fact, gravity is understood to be the curvature of spacetime, and since spacetime bends with matter, the intrinsic motion of space with respect to energy is always 0m/s. In this sense, gravitational acceleration is exclusively apparent motion.

Knowing that means you don't need to worry the next time you fall off a cliff. It only appears that way.